This is a Shannon award providing partial support for the research projects that fall short of the assigned Institute's funding range but are in the margin of excellence. The Shannon award is intended to provide support to test the feasibility of the approach; develop further tests and refine research techniques; perform secondary analysis of available data sets; or conduct discrete projects that can demonstrate the PI's research capabilities or lend additional weight to an already meritorious application. The abstract below is taken from the original document submitted by the principal investigator. DESCRIPTION: (Adapted From The Applicant's Abstract.) The anatomical and functional parcelling of sensory neocortex into distinct regions is heavily influenced by afferent axons originating in the thalamus. These findings as well as advances in anterograde and retrograde tracing methods have led to a reexamination of thalamocortical afferent terminations and their relationship to cortical laminar differentiation. In contrast to the visual and somatosensory systems, little is known concerning the morphology and distribution of specific classes of medial geniculate afferents to auditory cortex. Recent findings have revealed a patch-like organization of thalamo-cortical afferents arising from the vMGB. The afferent patches have an intermittent distribution resembling the patchy distribution of binaural interaction classes defined in physiological studies. In the tangential plane, the patches form elongated bands exceeding 2mm in length parallel to isofrequency contours. These results support a model of functionally distinct parallel vMGB projections to cortex. Aim 1 will determine the relationship between the thalamocortical patches and binaural interaction columns in normal rabbits, using physiological mapping in rabbits injected with biocytin in the MGB. Aim 2 will characterize the projections of the separate subdivisions of the MGB on the basis of laminar termination and spatial organization. Aim 3 is a continuation of experiments of Aim 2 with the addition of calcium binding protein immunohistochemistry. In aim 4, the developmental time-course for the arrival, growth and arborization of thalamocortical axons will be compared with the laminar differentiation of auditory cortex. The postnatal remodeling of thalamocortical arbors and their temporal relationship with dendritic resorbtion by target cells within laminae 3 and 4 will be examined. These studies will provide new information on the thalamocortical circuits subserving tonotopic and binaural maps and the morphological substrates for parallel MGB pathways to A1. They will also provide a morphological basis for understanding the trophic role of auditory afferents in cortical development and dendritic growth.